1. Field of the Invention
The present invention is directed to an electric assist steering system, and, more particularly, to a method and apparatus for detecting a failed thermistor in an electric assist steering system and controlling the electrical steering motor.
2. Description of the Related Art
Electric assist steering systems are well known in the art. In such electric assist steering systems, an electric assist motor, when energized, provides torque assist to aid the driver in turning steerable wheels of the vehicle. The electric assist motor is typically controlled in response to both steering torque applied to the vehicle steering wheel and measured vehicle speed. A controller monitors steering torque and controls a drive circuit to, in turn, control current applied to the electric assist motor. Such drive circuits typically include field effect transistors (FET's) or other forms of solid state switches operatively coupled between the vehicle battery and the electric assist motor. Current is controlled by pulse width modulation of the FET's or switches.
If steering torque is applied and the steerable wheels are held immovable, such as when the wheels are against a curb, the temperature of both the electric assist motor and the FET's rise. Such a condition is referred to as a stall condition of the electric assist motor. If the stall condition lasts for an extended period, the motor and/or FET's can overheat and fail.
U.S. Pat. No. 6,338,016 to Miller et. al. discloses an electric assist steering system that includes a stall detection apparatus for an electric assist steering system. A temperature sensor monitors the temperature of a power switch. A stall detector operatively reduces power to the electric assist motor to a respective power level for reducing the output torque of the electric assist motor in response to the respective temperature threshold measured.
U.S. Pat. No. 5,517,415 to Miller, which is assigned to TRW, Inc., discloses a stall detection apparatus for an electric assist steering system. A stall detector controls electrical current which is supplied to the electric motor in response to detecting the occurrence of a stall condition. This prevents damage to both the motor and drive circuitry.
U.S. Pat. No. 4,532,567 to Kade discloses an electric assist steering system which measures actual current through the motor. If the measured current exceeds a limit, the motor drive current is reduced.
U.S. Pat. No. 5,097,918 to Daido et al. discloses an electric assist steering system that includes an electric assist motor connected to a steering mechanism through an electromagnetic clutch. The system further includes a sensor for sensing rotational motion of the electric assist motor. When torque is sensed and the rotary sensor indicates that the motor is not rotating, the system disengages the electromagnetic clutch.
U.S. Pat. No. 4,878,004 to Shimizu discloses an electric assist steering system that includes an electric assist motor and a controller for controlling operation of the motor. The system further includes a current sensor which senses the magnitude of the actual current through the electric assist motor. An average value motor current is determined. The motor is controlled in response to the determined average current value.
The above references disclose methods for sensing current supplied to the electric motor for detecting a stall or over-use condition and adjusting the current level supplied to the electric motor for operating the electric motor at an operating level that prevents damage to the electric motor. One method used for detecting power dissipated by the electric motor and motor driving circuitry is to utilize a temperature sensor for monitoring the temperature of the power driving circuitry or the electric motor. Based on a respective temperature threshold reached, the current is reduced. However, none of the systems identify a method for detecting a failed temperature sensor which would result in the electric assist motor operating under full load when it is not desirable to do so. Under such conditions, if the temperature sensor is non-operational, damage to the system and loss of steering assist may occur.